1. Field of the Invention
The present invention relates to a cathode ray tube, and more particularly, to a cathode ray tube having a shadow mask for obviating doming phenomenon caused by thermal expansion and deterioration of color purity.
2. Discussion of the Related Art
FIG. 1 illustrates the structure of a color cathode ray tube according to a related art.
Referring to FIG. 1, the cathode ray tube includes a front side glass panel 1, and a rear side glass funnel 2 welded to the panel 1. The panel 1 and the funnel 2 are welded to each other in such a manner that their interior is in a vacuum state, thereby forming a vacuum tube.
A fluorescent screen 4 is formed on the inside surface of the panel 1, and an electron gun 8 is mounted on a neck portion 10 of the funnel 2 opposite of the fluorescent screen 4. A shadow mask 3 with a color selecting function is situated between the fluorescent screen 4 and the electron gun 8, maintaining a predetermined distance from the fluorescent screen 7. The shadow mask 3 is supported by a mask frame 14. Also, the mask frame 14 is elastically supported by a mask spring 5 and connected to a stud pin 6 to be supported by the panel 1.
The mask frame 14 is joined with an inner shield 7 made of a magnetic material. The inner shield 7 reduces the movement of an electron beam 11 due to external magnetic field during operation of the cathode ray tube. A deflection yoke 9 for deflecting the electron beam 11 emitted from the electron gun 8 is mounted into the neck portion 10 of the funnel 2. Also, a reinforcing band 12 is included in order to reinforce the front surface glass under the influence of the vacuum state inside the tube.
In operation, the electron beam 11 emitted from the electron gun 8 is deflected vertically and horizontally by the deflection yoke 9, and the deflected electron beam 11 passes through a beam passing hole on the shadow mask 3 and strikes the fluorescent screen 4 on the front, consequently displaying a desired color image.
FIG. 2 illustrates a shadow mask before it undergoes a press-forming process, and FIG. 3 illustrates the shadow mask of FIG. 2 after it undergoes a press-forming process.
Referring to FIGS. 2 and 3, a skirt 15 of the shadow mask 3 (before it undergoes the press-forming process) includes a slit 17 and a guide slit 18. The slit 17 serves to prevent the skirt 15 from being wrinkled and the guide slit 18 is used as a base position during the press-forming process of the shadow mask 3.
After the shadow mask 3 is press-formed, the skirt 15 is bent at right angles to a portion where beam passing holes are formed, and an embossment 16 is formed in order to promote formation of the skirt 15 and reinforce the strength of the skirt 15.
Then, the shadow mask 3 is fitted in the cathode ray tube by welding a welding portion 19 of the skirt 15 and the mask frame 14 together.
FIG. 4 is a schematic view illustrating a doming phenomenon in which the shadow mask 3 undergoes a deformation due to thermal expansion, and the electron beams 11 miss the intended target or mis-land on the fluorescent screen 4 because of the deformation. Some of the electron beams 11 do not pass through the beam passing holes of the shadow mask 3, and irradiate the inside surface of the shadow mask 3 instead. As a result, the shadow mask 3 is heated by the energy from the electron beams. As the temperature of the shadow mask 3 increases, the shadow mask is thermally expanded. Hence, the shadow mask 3 and the beam passing holes formed on the shadow mask 3 also undergo the thermal deformation. Subsequently, the trajectory of the electron beams 11 arriving at the fluorescent screen 4 is changed and the electron beams 11 mis-land.
FIG. 5 is a schematic view illustrating the thermal deformation of the mask frame and the shifting of the shadow mask position, eventually causing the electron beams to mis-land. The increased temperature (heat) of the shadow mask 3 through energy from the electron beams 11 is transferred to the mask frame 14, causing the mask frame 14 to thermally expand. This thermally expanded mask frame 14 then causes the shadow mask 3 to be displaced in the opposite direction of the original thermal displacement direction.
FIG. 6 is a diagram illustrating the degree of mis-landed electron beams due to the thermal deformation of the shadow mask and the mask frame. The thermal deformation depicted in FIGS. 4 and 5 changes the position of the shadow mask 3. At first, the degree of mis-landed electron beams increases when the shadow mask is thermally deformed as shown in FIG. 4. Then, the degree of mis-landed electron beams decreases when the mask frame 14 undergoes thermal deformation.
When the electron beams mis-land, color purity of the cathode ray tube deteriorates and it becomes difficult to correct the electron beams' landing problem.
To obviate this problem, a bimetal mask spring was used to compensate for thermal deformation of the mask frame, but this was not sufficient to solve the problem completely. Instead, the highly expensive mask spring only increased manufacturing cost of the cathode ray tube.